Scratch-resistant anticorrosion sol-gel coating for the protection of AZ31 magnesium alloy via a low temperature sol-gel route

A novel hybrid sol-gel/polyaniline coating has been developed for application onto an AZ31 magnesium alloy for corrosion protection. Electrochemical Impedance Spectroscopy in 3.5 wt% NaCl and diluted Harrison’s solutions, along with salt spray tests showed that the coating possesses excellent corrosion resistance. The hybrid coating was modified by doping with silica nanoparticles (for scratch resistance) and cured at a low temperature of 75 °C. Whilst conventional sol-gel methods tend to limit the coating thickness values up to 10 μm, the new hybrid sol-gel/polyaniline system presented here allows thick coatings to be deposited, in this case, around 50-60 μm.     

Study on the interface of PVDF coatings and HF-treated AZ31 magnesium alloy: Determination of interfacial interactions and reactions with self-healing properties

In this paper the interface of poly(vinylidene fluoride) coatings prepared by the dip coating method and HF-treated AZ31 magnesium alloy was evaluated. The best performance of this system in corrosion tests compared to ground, as-received and acetic acid cleaned substrates is related to an acid–base interaction at the interface and to interfacial reactions which resulted in a self-healing process. The protectiveness of the samples was investigated using impedance and immersion tests while the coating morphology and interface stability were investigated by scanning electron microscopy, X-ray photoelectron spectroscopy and adhesion tests.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

AZ31B镁合金表面微弧氧化-溶胶凝胶复合膜层干、湿热交替条件下耐蚀性研究

采用微弧氧化与溶胶凝胶技术在AZ31B镁合金表面制备微弧氧化-溶胶凝胶复合膜层,并于干、湿热交替盐雾腐蚀环境下,对复合膜层的耐蚀性进行研究。结果表明,随腐蚀时间的延长,复合膜层宏观表面出现白色斑点,且斑点数量逐渐增多。微观形貌分析表明,试样原始复合膜层表面存在一定数量的微裂纹及孔洞,随干、湿热交替盐雾腐蚀的进行,复合膜层表面局部应力状态发生变化,膜层表面微裂纹及孔洞减少、减小,裂纹边缘处复合膜层有轻微剥落现象。在干、湿热交替腐蚀环境中,复合膜层划格实验评级结果均为0级,膜基结合状态良好;电化学分析表明,在腐蚀实验过程中腐蚀电位未发生明显改变,均保持在约-1.43 V,复合膜层具有良好的耐蚀性。     

镁合金微弧氧化膜层退除工艺

目的提出一种镁合金微弧氧化膜层的退除工艺,提高镁合金的二次利用率。方法以硝酸、氟化钾、柠檬酸、十二烷基苯磺酸钠(SDBS)及缓蚀剂为组分配制退膜液,设计正交试验,以退除速率、表面粗糙度作为评判标准,优化退膜液配方。分析退膜液中各组分的作用,研究退膜过程中退膜量与时间的关系,讨论膜层厚度与腐蚀率、表面粗糙度的关系。采用优化的退膜液对镁合金微弧氧化膜层进行退除,观察表面宏观及微观形貌。结果退膜液各组分针对退膜速率和退膜后镁合金基体表面粗糙度的极差由大到小均依次为:R硝酸R氟化钾R柠檬酸RSDBSR缓蚀剂。对退膜速率和表面粗糙度影响最大的是硝酸浓度,其次是氟化钾浓度,柠檬酸、SDBS及缓蚀剂浓度的影响最小。在整个退膜过程中,膜层退除量与退膜时间并不呈线性关系。退膜开始阶段及完成阶段,膜层退除量大,退膜速率高;退膜中期,膜层退除量与退膜时间基本呈线性关系,且退膜速率小于初始退膜速率。XRD分析显示,退膜后的镁合金表面无残余腐蚀产物。二次微弧氧化膜层的SEM图像显示,微孔结构致密,分布均匀,与一次微弧氧化的膜层无明显差别。结论镁合金微弧氧化膜层退除液的最佳配方为:硝酸90 m L/L,柠檬酸8 g/L,氟化钾35g/L,十二烷基苯磺酸钠5 g/L,缓蚀剂6.5 g/L。该退膜液退膜效果良好,对镁合金基体损伤小,退膜速率快,成本低廉,可用于不合格镁合金零部件微弧氧化膜层的多次退除返修。     

Characterization and corrosion studies of ceria thin film based on fluorinated AZ91D magnesium alloy

The CeO₂ thin film was prepared via sol-gel method on fluorinated AZ91D magnesium alloy surfaces. The surface morphology, composition and the corrosion resistance of the film were investigated in details using scanning electron microscope, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy as well as potentiodynamic polarization tests. It was found that small amount of MgO and MgF₂ were encapsulated in CeO₂ thin film. The electrochemical measurement results demonstrated that the CeO₂ thin film on fluorinated AZ91D magnesium alloy could improve the corrosion resistance approximately by two orders of magnitude compared with that of the bare substrate.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

Influence of inorganic acid pickling on the corrosion resistance of magnesium alloy AZ31 sheet

Surface contaminants as a result of thermo-mechanical processing of magnesium alloys, e.g. sheet rolling, can have a negative effect on the corrosion resistance of magnesium alloys. Especially contaminants such as Fe, Ni and Cu, left on the surface of magnesium alloys result in the formation of micro-galvanic couples and can therefore increase corrosion attack on these alloys. Due to this influence they should be removed to obtain good corrosion resistance.In this study, the effect of inorganic acid pickling on the corrosion behaviour of a commercial AZ31 magnesium alloy sheet was investigated. Sulphuric, nitric and phosphoric acids of different concentrations were used to clean the alloy for various pickling times. The surface morphology, composition and phases were elucidated using scanning electron microscopy, X-ray fluorescence analysis, spark discharge-optical emission spectroscopy, energy dispersive X-ray spectroscopy and infrared spectroscopy. The effect of surface cleaning on the corrosion properties was studied using salt spray test and electrochemical impedance spectroscopy. The experimental results show that acid pickling reduces the surface impurities and therefore enhances the corrosion resistance of the alloy. The cleaning efficiency of the three acids used and the corrosion protection mechanisms were found to be remarkably different. Best corrosion results were obtained with nitric acid, followed closely by phosphoric acid. Only the sulphuric acid failed more or less when cleaning the AZ31 sheet. However, to obtain reasonable corrosion resistance at least 5 μm of the surface of AZ31 magnesium alloy sheet have to be removed.     

前处理工艺对镁合金钛/锆转化膜耐蚀性能的影响

研究了酸洗以及酸洗+碱洗前处理工艺对AZ91D镁合金无铬、无裂纹、低能耗钛/锆转化膜耐蚀性能的影响。结果表明,单独的酸洗前处理使得AZ91D镁合金表面的α相优先溶解,合金表面粗糙度增加,不利于钛/锆转化膜耐蚀性能的增加。合理地利用酸洗+碱洗调整AZ91D镁合金表面化学状态能够有效提高钛/锆化学转化膜的耐蚀性能。     

The effect of epoxy coating containing emeraldine base and hydrofluoric acid doped polyaniline on the corrosion protection of AZ91D magnesium alloy

Corrosion protection of epoxy coatings containing emeraldine base polyaniline (PANI) or hydrofluoric acid doped PANI on AZ91D magnesium alloy were studied by EIS and Pull-Off Adhesion Test. The results indicated that the addition of emeraldine base PANI or hydrofluoric acid doped PANI could improve the corrosion resistance of epoxy coating. The epoxy coating containing hydrofluoric acid doped PANI had the best performance of the corrosion protection among three systems under investigation. The corrosion product film was analyzed by XPS indicating that PANI changed the chemical structure of the corrosion film. The protective mechanism imparted by PANI was discussed.     

Sol-gel silica coatings on ZE41 magnesium alloy for corrosion protection

Silica coatings have been applied on the surface of ZE41 magnesium alloy following the organic sol-gel route and the dip-coating technique. Three different concentrations of sol solution and two densification temperatures of the coating (400 °C and 500 °C) were used to optimize the compaction of the coatings and as a result reach the corrosion protection of the metallic substrate tests in 3.5 wt.% NaCl aqueous solution. Crack-free coatings with thickness in the 2-3 μm were obtained on the ZE41 magnesium alloy. The combination of high alkoxide concentration in the sol-gel formulation, and the high sintering temperature (500 °C) leads to coating (D500) with the optimal physical barrier against the corrosion process. This coating was capable of resisting more than 7 days in contact with the aggressive electrolyte suffering minor corrosion degradation. A corrosion mechanism for each of the tested specimens has been proposed.     

The effects of 8-hydroxyquinoline on corrosion performance of a Mg-rich coating on AZ91D magnesium alloy

A Mg-HQ-rich primer was prepared by adding 45% pure magnesium particles and 5% 8-hydroxyquinoline in epoxy coating. The Mg-HQ-rich primer showed obviously better protection for AZ91D alloy than 50% Mg-rich primer. The additive 8-HQ combines with Mg²⁺ in the coating, forming a insoluble complex, MgQ₂, instead of Mg(OH)₂ on Mg particles. The MgQ₂ layer may retard dissolution of magnesium particles and prolong the cathodic protection effect of Mg-HQ-rich primer. Additionally, the corrosion products may also block the defects in the primer and the active sites on the substrate surface, thereby further increase the corrosion resistance of the Mg-HQ-rich primer.     

Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy

A pure Al coating was deposited on AZ91D magnesium alloy through cold spray (CS) technique. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the grain interfaces and subgrains formed close to the particle/particle boundaries. Electrochemical tests revealed that the cold sprayed pure Al coating had better pitting corrosion resistance than bulk pure Al with similar purity in neutral 3.5 wt.% NaCl solution. In addition, a mass-transfer step was found to be involved in the corrosion during 10 days immersion.     

Preparation of Phytic Acid/Silane Hybrid Coating on Magnesium Alloy and Its Corrosion Resistance in Simulated Body Fluid

In order to decrease the corrosion rate and improve the bioactivity of magnesium alloy, phytic acid/saline hybrid coatings were synthesized on AZ31 magnesium alloys by sol–gel dip-coating method. It was found that the mole ratio of phytic acid to γ-APS had a great influence on coating morphology and the corresponding corrosion resistance of the coated magnesium alloys. When the mole ratio of phytic acid to γ-APS was 1:1, the obtained hybrid coating was integral and without cracks, which was ascribed to the strong chelate capability of phytic acid and Si-O-Si network derived from silane. Electrochemical test result indicated that the corrosion resistance of the coated magnesium alloy was about 27 times larger than that of the naked counterpart. In parallel, immersion test showed that the phytic acid/silane hybrid coating could induce CaP-mineralized product deposition, which offered another protection for magnesium alloy.     

Corrosion protection of magnesium AZ31 alloy using poly(ether imide) [PEI] coatings prepared by the dip coating method: Influence of solvent and substrate pre-treatment

In this study, investigations on the protectiveness of poly(ether imide) coatings against corrosion of magnesium AZ31 alloy sheets are performed. The coatings were prepared in different pre-treated substrates by the dip coating method using N′N′-dimethyl acetamide (DMAc) and N′-methyl pyrrolidone solutions. The optimal performance was obtained for hydrofluoric acid treated substrates coated using DMAc solution (coating thickness 13 μm) which showed impedances in the order of 10⁷ Ω cm² even after more than 3300 h of exposure to a 3.5 wt.% NaCl solution. This high performance is associated to an acid–base interaction at the interface as observed by X-ray photoelectron spectroscopy.     

MB8镁合金植酸转化膜的制备及性能

通过单因素实验优化镁合金植酸转化工艺,发现在植酸体积浓度为10 mL/L,pH=2.5,温度50℃,成膜时间20 min条件下,可在MB8镁合金表面制备出均匀一致的植酸转化膜.该膜层微观形貌与铬酸盐转化膜类似,表面呈现出均匀分布的网状微裂纹,类似于"龟裂的土地",膜层主要成分为Mg,O,P,Mn和C;动电位极化曲线测试...     

Effect of magnesium hydride on the corrosion behavior of an AZ91 magnesium alloy in sodium chloride solution

The effect of magnesium hydride on the corrosion behavior of an as-cast AZ91 alloy in 3.5 wt.% NaCl solution was investigated using gas collection method and potentiostatic test. The Pourbaix diagram of Mg–H₂O system was built using thermodynamic calculation. It was possible that magnesium hydride could form in the whole pH range in theory. The experimental results showed that at cathodic region, magnesium hydride formed on surface, which was the controlling process for the corrosion behavior of AZ91 alloy; at anodic region and free corrosion potential, magnesium hydride model and partially protective film model, monovalent magnesium ion model and particle undermining model were responsible for the corrosion process of AZ91 alloy.     

pH值对AZ91D镁合金表面植酸转化膜的影响

在pH值不同的几种植酸转化液中对AZ91D镁合金进行表面转化处理,利用扫描电镜及自带能谱仪、Tafel曲线和阻抗谱等分析手段,结合NaCl水溶液点滴实验,研究了pH值对植酸转化膜表面成分及耐蚀性的影响.结果表明:在酸性条件下形成的植酸转化膜的耐蚀性较好;转化液的pH值会影响植酸螯合物的形成和转化膜的物质组成,进而影响转化膜的耐蚀性能;在酸性条件下,镁合金表面形成的植酸转化膜应属于电子导体膜,它阻碍了腐蚀介质与基体的接触,同时抑制腐蚀产物的扩散,对镁合金起到防护作用.     

Electroplated super-hydrophobic Zn-Fe coating for corrosion protection on magnesium alloy

A superhydrophobic Zn-Fe alloy coating was prepared on the surface of a reactive magnesium alloy using a simple, low-cost, eco-friendly method. Firstly, the Zn-Fe coating was obtained in a neutral glycerol Zn-Fe plating solution, which is green, compositionally stable, and non-corrosive to the equipment. And then the superhydrophobic surface with a flower-like microstructure was obtained by grafting myristic acid onto the Zn-Fe coating via a chelation reaction. The water contact angle was >150° and the rolling angle was 3°-4°. The corrosion rate of the two groups of superhydrophobic magnesium alloy samples with electrodeposition time of 30 and 50 min, respectively, was reduced by about 87% compared to that of the bare magnesium alloy. The prepared superhydrophobic coatings exhibit high performance in self-cleaning, abrasion resistance, and corrosion resistance.     

Corrosion of zinc–magnesium coatings: Mechanism of paint delamination

Due to their promising corrosion properties, metallic coatings containing magnesium are currently widely investigated for use as protective coatings for steel sheet. Particularly, alloying zinc coatings with magnesium results in a remarkable improvement of the corrosion resistance of the painted system. While some aspects of this improvement have been understood, the progress of the corrosive degradation of the alloy coating/paint interface has not been reported in detail. In this paper, the delamination of a model polymer from the intermetallic MgZn₂ is described and a degradation mechanism proposed. Aspects for the design of stable interfaces are discussed.